Embossing is carried out by passing a web between two or more embossing rolls, at least one of which carries the desired emboss pattern. Known embossing configurations include rigid-to-resilient embossing and rigid-to-rigid embossing. In a rigid-to-resilient embossing system, a single or multi-ply substrate is passed through a nip formed between a roll whose substantially rigid surface contains the embossing pattern as a multiplicity of protuberances and/or depressions arranged in an aesthetically-pleasing manner, and a second roll, whose substantially resilient surface can be either smooth or also contain a multiplicity of protuberances and/or depressions which cooperate with the rigid surfaced patterned roll. Commonly, rigid rolls are formed with a steel body which is either directly engraved upon or which can contain a hard rubber-covered, or other suitable polymer, surface (directly coated or sleeved) upon which the embossing pattern is formed by any convenient method such as, for example, being laser engraved. The resilient roll may consist of a steel core provided with a resilient surface, such as being directly covered or sleeved with a resilient material such as rubber, or other suitable polymer. The rubber coating may be either smooth or engraved with a pattern. The pattern on the resilient roll may be either a mated or a non-mated pattern with respect to the pattern carried on the rigid roll.
In a rigid-to-rigid embossing process, a single-ply or multi-ply substrate is passed through a nip formed between two substantially rigid rolls. The surfaces of both rolls contain the pattern to be embossed as a multiplicity of protuberances and/or depressions arranged into an aesthetically-pleasing manner where the protuberances and/or depressions in the second roll cooperate with those patterned in the first rigid roll. The first rigid roll may be formed, for example, with a steel body which is either directly engraved upon or which can contain a hard rubber-covered, or other suitable polymer, surface (directly coated or sleeved) upon which the embossing pattern is engraved by any conventional method, such as by laser engraving. The second rigid roll can be formed with a steel body or can contain a hard rubber covered, or other suitable polymer, surface (directly coated or sleeved) upon which any convenient pattern, such as a matching or mated pattern, is conventionally engraved or laser-engraved. In perforate embossing, a rigid-to-rigid embossing system is typically used. However, a rigid-resilient configuration can also be used for perforate embossing. In a related operation, normally referred to as “Dry Marking”, the pattern is formed by protrusions on one roll which compress the sheet against an anvil roll which is normally smooth surfaced.
In perforate embossing the embossing elements are configured such that at least a portion of the web located between the embossing elements is perforated. As used herein, generally the terminology “perforated”, “perforate” and the like refers to the existence of either (1) a macro-scale through aperture in the web or (2) when a macro-scale through aperture does not exist, at least incipient tearing such as would increase the transmittivity of light through a small region of the web or would decrease the machine direction strength of a web by at least 15% for a given range of embossing depths. When the degree of incipient tearing is controlled such that the loss of MD strength is less than about 15% but increased transmittivity is obtained, we prefer that the loss of MD strength is at least about 10%. In many cases, it will be advantageous to perf-emboss heavily such that the MD tensile strength is decreased about 35% to about 65%.
Commonly absorbent products such as tissue or towel are subjected to various combinations of both calendering and embossing to bring the softness and bulk parameters into acceptable ranges for premium quality products. Calendering adversely affects bulk and may raise tensile modulus, which is inversely related to tissue softness. Embossing increases product caliper (bulk) and can reduce modulus, but lowers strength and can have a deleterious effect on surface softness. Accordingly, it can be appreciated that these processes can have both beneficial and adverse effects on strength, appearance, surface smoothness and particularly thickness perception since there is a fundamental conflict between bulk and calendering.
Cross-machine direction (CD) tensile strength and stretch can be associated with consumer preference for absorbent paper products such as paper toweling. In particular, consumers prefer a strong, yet balanced, towel of which cross-machine direction (CD) strength and stretch and machine direction strength and stretch are components. Because un-embossed basesheet is typically much stronger and has more stretch in the machine direction than the cross-machine direction, an embossing process which does not lead to excessive losses in cross-machine direction tensile strength or stretch is desirable for absorbent sheet and more particularly for sheet which has machine direction ridges as described herein.
In some through air (TAD) processes, an overall pattern is imparted to the web during the forming and drying process by use of a patterned fabric having designs to enhance appearance, cross direction stretch and to balance properties. Such features may include ridges extending in the machine direction. Through air dried tissues can be deficient in surface smoothness and softness unless strategies such as calendering, embossing, chemical softeners and stratification of low coarseness fibers on the tissue's outer layers are typically employed in addition to creping.
In U.S. Pat. Nos. 5,656,134; 5,690,788; 5,685,954; 6,096,168; and 5,885,415 to Marinack et al. (hereinafter the Marinack et al. patents), the disclosure of which is incorporated by reference it was shown that paper products having highly desirable bulk, appearance (including reflectivity) and softness characteristics, can be produced by a process similar to conventional wet-press (CWP) processes by replacing the conventional creping blade with an undulatory creping blade having a multiplicity of serrulated creping sections presenting differentiated creping and rake angles to the sheet. Further, in addition to imparting desirable initial characteristics directly to the sheet, the process of the Marinack et al. patents produces a sheet which is more capable of withstanding calendering without excessive degradation than a conventional wet pressed tissue web.
The process and apparatus of the Marinack et al. patents makes it possible to achieve surprisingly high absorbency in a relatively non-bulky towel thus providing an important new benefit. Similarly, webs made by way of undulatory creping can be calendered more heavily than many conventional webs while still retaining bulk and absorbency, making it possible to provide smoother, and thereby softer feeling surfaces without unduly increasing tensile modulus or unduly degrading bulk. On the other hand, if the primary goal is to save on the cost of raw materials, the tissue of the Marinack et al. patents can have surprising bulk at a low basis weight without an excessive sacrifice in strength or at low percent crepe while maintaining high caliper. Creping in accordance with the Marinack et al. patents creates a machine direction oriented shaped sheet which has higher than normal stretch in directions other than the machine direction, that is, particularly high cross-direction stretch. Embossing without the desired protocol can negate the gains realized by using the undulatory creping process and/or the benefits of molded-in machine direction ridges in the web. There is provided in accordance with the present invention creping protocols and products which enhance properties while preserving the benefits imparted to the web during its manufacture by incorporating ridges extending in the machine direction.